Update docs for incident specific slack notifs

What does this MR do and why?

Add/change documentation for new notification setting developed as part of #367550 (closed)

Screenshots or screen recordings

NA

How to set up and validate locally

NA

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #367550 (closed)